1. Field of the Invention
The present invention relates to an image forming apparatus, a color deviation correcting method, and its program and, more particularly, to a mechanism for predicting a deviation amount regarding a laser beam irradiating position in an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus for forming a color image by overlaying toner images of a plurality of colors, from a viewpoint of quality of a product, it is important that the image of each color is accurately printed at a predetermined position on a printed matter, that is, no color deviation occurs. There are various factors of the color deviation. As one of the factors which exert a large influence, there is a fluctuation in laser beam irradiating position on a photosensitive material which occurs in association with a thermal deformation of an optical unit.
A description will be made with reference to a schematic cross sectional view of an optical unit 10 illustrated in FIG. 14. Generally, the optical unit has such construction that a laser beam irradiated from each of light emitting sources 56 (optical elements) is reflected by a rotating polygon mirror 57, thereby scanning. The laser beam is reflected by a mirror a few times for a time interval until the laser beam emitted from each light emitting source 56 reaches each photosensitive drum 12, a progressing direction of the laser beam is changed, and a spot and a scanning width can be adjusted through a lens.
Those mechanical elements which decide an optical path L of the laser beam are fixed to a frame which forms the optical unit 10. When a thermal deformation occurs in the frame by a temperature elevation associated with the operation of the image forming apparatus, positions of those elements also change and a direction of the laser optical path L changes. The change in direction of the optical path is increased in proportion to an optical path length until the laser beam reaches the photosensitive drum 12. Therefore, even if the frame deformation of the optical unit 10 is very small, the change in optical path direction appears as a fluctuation of a laser beam irradiating position 53. The fluctuation of the laser beam irradiating position associated with such a temperature elevation phenomenon is called a thermal shift of the laser beam irradiating position.
An intra-apparatus temperature elevation of the image forming apparatus (intra-machine temperature elevation), and a self temperature elevation of the optical unit 10 caused by heat generation of a motor for driving the polygon mirror 57 have been recognized hitherto as factors which cause the laser beam irradiating position to fluctuate.
As a method of certainly correcting the color deviation caused by those factors, there is a method of performing a calibration for matching laser beam irradiating timing of each color. The calibration denotes such a process that a calibration pattern is formed onto an intermediate transfer material by a toner image and by reading the calibration pattern by a sensor, writing timing of each color is made coincident. However, the calibration cannot be frequently performed in consideration of a time necessary for the calibration, an influence on lives of consumables as well as toner consumption, and the like.
Therefore, a method whereby a temperature sensor and a color deviation correcting unit for presuming the fluctuation in laser beam irradiating position based on an output of the temperature sensor and correcting the laser beam irradiating timing are provided, and the color deviation is corrected without performing the calibration has been proposed. As a typical construction for realizing such a method, there is such a construction that a temperature of the optical unit itself is detected by a temperature sensor and the laser beam irradiating position is corrected by a correction control unit (for example, refer to Japanese Patent Application Laid-Open No. 2000-218860). There are also such a construction that an intra-apparatus temperature is detected by a temperature sensor and the laser beam irradiating position is corrected by a correction control unit based on a detection result and the like (for example, refer to Japanese Patent Application Laid-Open No. 2003-207976 and Japanese Patent Application Laid-Open No. 2005-234099).
Those methods are based on such a design idea that the temperature of a portion which exerts an influence on the thermal shift is measured and the laser beam irradiating position is corrected according to a temperature change. In the case where the temperature change and a color deviation tendency can be approximated by a one-to-one correspondence relation, it can be said that such a method is a very simple correcting method. FIG. 15A illustrates a graph of a case where a correlation between the change in laser beam irradiating position and the temperature change can be found out. A curve drawn by a thin solid line indicates a time-dependent change of the intra-apparatus temperature. A curve drawn by a bold solid line indicates a time-dependent change of the laser beam irradiating position fluctuation.
However, in the actual product, for a time interval until the color deviation fluctuation is converged, a correlation between a fluctuation increase or decrease in the laser beam irradiating position and a temperature increase or decrease cannot be always easily found out. The fluctuation increase and the fluctuation decrease mentioned here denote a relative relation and if any one direction is assumed to be an increase direction, the other is set to a decrease direction. The conventional color deviation correcting method or predicting method of the laser beam irradiating position cannot be applied to such a case. As a specific example, a case where in spite of the fact that the temperature of the portion which exerts an influence on the thermal shift is continuously rising, the direction of the color deviation fluctuation is reversed from the forward direction to the reverse direction or vice versa in such a step corresponds to such a case. FIG. 15B illustrates a graph of a case where it is difficult to find out a correlation between the change in laser beam irradiating position (deviation according to the temperature change) and the temperature change. A thin solid line indicates a time-dependent change of the intra-apparatus temperature. A bold solid line indicates a time-dependent change of the laser beam irradiating position fluctuation. It is difficult to find out a correlation between the fluctuation tendency of the laser beam irradiating position and the temperature change of a portion around the optical unit.
It is considered that the construction of the image forming unit of the conventional image forming apparatus is related to a background in which the predicting method mentioned in the related art has been used. Hitherto, a construction in which the optical unit which can scan only one laser beam is exclusively provided for every color as a light source of the laser beam necessary to form a color image is a mainstream. Since such a construction of the optical unit is simple, the temperature change and the fluctuation of the laser beam irradiating position can be easily approximated by the one-to-one relation. Since the optical unit of the same construction is used every color, the thermal shifts of the laser beam irradiating positions exhibit a similar tendency in all colors and a relative difference between the colors can be easily found out.